There have conventionally been used analog signal input devices that adopt an inexpensive circuit system, in which an insulation transformer or the like is employed as insulation means to collect analog signals outputted from an external device such as a sensor including a thermocouple. Installation environments of the thermocouple or the like are generally inadequate, so disconnection detection means for generating a test voltage for detecting a disconnection and a contact failure caused in the external device such as the thermocouple is mounted to those analog input devices.
As the conventional analog input devices, for example, there are disclosed ones which include a power supply section for generating a test voltage, and employs a method in which a signal is inputted through an insulation transformer from a control circuit and the test voltage generated thereby is applied to a thermocouple to detect a disconnection state (for example, Patent Documents 1 and 2).
Configuration examples of this kind of conventional analog input device are shown in FIGS. 5 and 6. FIG. 5 is a configuration diagram showing a conventional analog input device 101 mounted with a plurality of analog insulation multiplexers 111. As shown in FIG. 5, the analog input device 101 includes the plurality of analog insulation multiplexers 111 for insulating and collecting analog signals 109, a plurality of amplifiers 105 connected to the respective analog insulation multiplexers 111, a channel selector 106 which is connected to the amplifiers 105 and selects an input channel, an A/D converter 107 connected to the channel selector 106, and a control circuit 108 connected to the A/D converter 107. Further, each analog insulation multiplexer 111 is connected to a single gate element 1411 and a single gate element 1421. Note that each of the gate elements 1411 and 1421 is connected between the control circuit 108 and the analog insulation multiplexers 111.
FIG. 6 is a configuration diagram showing an internal circuit of the conventional analog insulation multiplexer 111 shown in FIG. 5. For simplification, in FIG. 6, the analog insulation multiplexer 111, the amplifier 105, and the gate elements 1411 and 1421 are shown one each only. In the analog insulation multiplexer 111, a power supply section 102 and a scanning circuit 103 are provided. Further, in the analog insulation multiplexer 111, a second insulation transformer T102 connected between the control circuit 108 and the scanning circuit 103 and a third insulation transformer T103 connected between the control circuit 108 and the power supply section 102 are provided.
The scanning circuit 103 for insulating and collecting the analog signals 109 outputted from the sensor such as the thermocouple includes a first insulation transformer T101 and a switching element S101. The analog signal 109 is insulated by the first insulation transformer T101 and inputted to the internal circuit of the analog input device 101. Further, a drive pulse 141 for performing an ON/OFF control of the switching element S101 is inputted to the second insulation transformer T102 from the control circuit 108 through the gate element 1411 and insulated by the second insulation transformer T102 to be transmitted to the switching element S101.
On the other hand, the power supply section 102 for detecting a disconnection and a contact failure of the sensor such as the thermocouple is connected to a secondary side of a third insulation transformer T103. A power supply pulse 142 inputted from the control circuit 108 through the gate element 1421 is applied to a primary side of the third insulation transformer T103, whereby a smoothing circuit 120 provided in the power supply section 102 is charged with a test voltage used for detecting a disconnection or the like. The test voltage is applied to an interface of the sensor such as the thermocouple through a high-resistor 121 provided in the power supply section 102. Note that, although omitted in FIGS. 5 and 6, voltage measurement means is provided. The voltage measurement means detects an abnormality such as a disconnection by detecting a voltage led to an output terminal of the sensor such as the thermocouple by the using the test voltage when the test voltage is applied, and comparing the voltage with a preset reference voltage, or the like.
Patent Document 1: JP 63-113622 A
Patent Document 2: JP 07-244561 A